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The water hammer issue is expansion as you have described. When a faucet is open, the pressure goes down - close the faucet, the pressure goes up (sorry for sounding like Dr. Phil). Something has got to give when the pressure goes back up. Usually the copper flexes and that's fine. You said you had 3/4" in there? That's pretty stiff. Even if you encased the copper plumbing in cement, something would need to give. It should be 1/2" copper IMHO.

If it were the utility, the pressure would gradually rise until the T&P blew off. There'd be some extreme pressure coming out of the taps as well. If the T&P hasn't gone off already, I'd look elsewhere.

Sometimes, with a tank having a side inlet and outlet, the air bubble in the top of the tank acts as a mini expansion tank. If that was the case previous, the problem would have been pre-existing but didn't show until the tank change. Going to a top inlet/outlet style tank would remove more of that air bubble and 'cause hammer.

If you're close to the meter after moving the tank, you may wish to consider adding for expansion - a long length of 1/2" copper prior to the inlet. I think 3/4" copper to a tank close to the meter would be a problem....but that's just me.

Thanks for the replies. No luck yet, but I haven't had a chance to really tackle it either.

I did notice that it sometimes hammers when a faucet is turned ON. Not every time and not every faucet. Usually flow is reduced, then a bang, then full flow. There is a problem somewhere, just got to find it

Thanks for the replies. No luck yet, but I haven't had a chance to really tackle it either.

I did notice that it sometimes hammers when a faucet is turned ON. Not every time and not every faucet. Usually flow is reduced, then a bang, then full flow. There is a problem somewhere, just got to find it

I didn't realize it was hammering while ON...it makes a difference.

Someone mentioned it already but I don't recall an answer. Did you verify if the inlet/outlet was correct? What's the temperature setting on the tank? Is there diminished capacity (does your customer run out of hot water)? Can you put your hand on the T&P valve without removing it for 7 seconds? Does the T&P feel lukewarm? Does the problem still happen when the tank isn't being heated? Does it still happen after draining a gallon out the T&P?

Lots of questions......keep those questions in mind when you take a look at it next time.

The water hammer issue is expansion as you have described. When a faucet is open, the pressure goes down - close the faucet, the pressure goes up (sorry for sounding like Dr. Phil). Something has got to give when the pressure goes back up. Usually the copper flexes and that's fine. You said you had 3/4" in there? That's pretty stiff. Even if you encased the copper plumbing in cement, something would need to give. It should be 1/2" copper IMHO.

If it were the utility, the pressure would gradually rise until the T&amp;P blew off. There'd be some extreme pressure coming out of the taps as well. If the T&amp;P hasn't gone off already, I'd look elsewhere.

Sometimes, with a tank having a side inlet and outlet, the air bubble in the top of the tank acts as a mini expansion tank. If that was the case previous, the problem would have been pre-existing but didn't show until the tank change. Going to a top inlet/outlet style tank would remove more of that air bubble and 'cause hammer.

If you're close to the meter after moving the tank, you may wish to consider adding for expansion - a long length of 1/2" copper prior to the inlet. I think 3/4" copper to a tank close to the meter would be a problem....but that's just me.

That is really interesting

Definitely no water out of the relief, but I did make an extra effort to ensure that the entire main run was 3/4". It never reduces to 1/2 until it branches to fixtures. But every fixture branch is 1/2

Tank was actually moved further from water meter, but a stub if 1/2 was removed as it was no longer required, I wonder if that's causing a problem

That gave me some great ideas to ponder, thanks.

I write my refrigeration CFQ tomorrow, so the plumbing got put on hold for a bit, but I will keep you posted

Someone mentioned it already but I don't recall an answer. Did you verify if the inlet/outlet was correct? What's the temperature setting on the tank? Is there diminished capacity (does your customer run out of hot water)? Can you put your hand on the T&amp;P valve without removing it for 7 seconds? Does the T&amp;P feel lukewarm? Does the problem still happen when the tank isn't being heated? Does it still happen after draining a gallon out the T&amp;P?

Lots of questions......keep those questions in mind when you take a look at it next time.

I did verify inlet and outlet are correct. However I have not had a chance to verify placement if dip tube yet

No temp display on control, but the digital display is set to the low end of the "comfort zone". Water is however considerably hotter than the setting on the previous tank, usually temps are in the 130 range. No Mixing valve

It sounds like the flow from the main is not keeping up with the flow from the branch to the fixture and then catches up with a bang. Just a thought, don't know why this could happen. Is the hammer always after the fixture is turned on or does it hammer sometimes when it is turned off?

Originally Posted by LKJoel

Thanks for the replies. No luck yet, but I haven't had a chance to really tackle it either.

I did notice that it sometimes hammers when a faucet is turned ON. Not every time and not every faucet. Usually flow is reduced, then a bang, then full flow. There is a problem somewhere, just got to find it

I did verify inlet and outlet are correct. However I have not had a chance to verify placement if dip tube yet

No temp display on control, but the digital display is set to the low end of the "comfort zone". Water is however considerably hotter than the setting on the previous tank, usually temps are in the 130 range. No Mixing valve

Never ran out of hot water on the old one or new one

The temperature at the T&P is an important measurement so you can compare the tank setting at the [control] near the bottom to the actual temperature by the T&P near the top of the tank. Record that temperature and then run the hot water faucet in the [basement laundry tub?] and time how long it takes for the heater to kick on. While waiting, measure the water temperature and watch for a temperature fluctuation - record temps every 30s. If the probe for the control is inserted 1/4 way up the tank and its a 60Gal, you will need to drain no more than 15Gal before it comes on. 2-3 minutes sounds about right if you're draining into a laundry tub.

Scenario #1 - If there no dip tube at all, you'll get "short cycling" and the T&P temperature will go from extreme scorching hot to very cold in 2-3 minutes and the heater will not come on for a long time - maybe 10-15 minutes. The capacity would be reduced in the water heater and the customer would crank up the temperature to near boiler temperatures (see#3).

Scenario #2 - If they are reversed internally, the tank may seem to operate normal but will have a compressed air bubble inside the top of the tank that could cause hammer. In this scenario, it might take 10 minutes for the heater to cycle.....I call it "reverse stacking". The heater would need to go through 45Gal of hot water (as opposed to 15Gal) before cycling. The T&P temperature will go from lukewarm to very cold in 2-3 minutes.

Scenario #3 - You have extreme boiler-like temperatures and lots of air is dropping out of water. Some H2O has a lot of O2.

Scenario #4 - Extreme pressure build up but not beyond 150psig. Something like a backcheck valve that was missed.

There are other scenario's, but these ones cover off most the issues you described.